This invention relates to a continuously monitoring apparatus to detect combustible vapors, such as natural gas leaks in homes and industry.
A brief review of local headlines during the past few years shows an increasing incidence of natural gas explosions with resulting loss of life, personal injury and extensive property damage. Study of the incidents leads one to the conclusion that many of these incidents could have been prevented if there was early detection. Natural gas, being virtually odorless, is difficult to detect, and by the time one does smell it, as the newspaper stories indicate, explosion is seconds away. This is true even though an odor bearing compound has been added to the gas so that it may be detected more easily by smell. Many leaks occur outside the house and those inside cannot smell it since the sulphur compound gets absorbed by the soil before it enters the house. The force of a natural gas explosion is awesome, not only is one house leveled but entire blocks have been obliterated. Causes of the leaks may vary from faulty equipment within a dwelling to gas mains broken during street construction to very old corroded and leaky gas mains. Regardless of the cause, the result is inevitably the same: the loss of life and widespread property damage.
The ideal solution is equipment that never fails and an absence of accidental puncturing of gas mains. Realistically, these goals are not attainable. Detection of the leak then becomes of prime importance. Early detection of such leaks allows the occupants of a house with a leak and their neighbors to clear the area and allows time for emergency crews to shut off the supply of gas in the vicinity of the leak.
Various devices are available for detection of gas leaks. Mine Safety Appliances Co. has a carbon monoxide alarm which is primarily intended for industrial use. The device utilizes a charcoal scrubber to clear the air sample and a hopcalite bed and an inactive chemical bed to measure the parts per million (PPM) of combustable gas in the surrounding air. When the PPM of carbon monoxide (CO) reaches a predetermined level, the device gives an alarm. The same company also manufactures other detection equipment. This type of device has a high degree of thermal inertia, i.e., slow response time. The chemical bed, e.g., hopcalite, is effected by temperature cycling, moisture and poisoning. This type of device needs frequent readjustment.
Many of the prior art devices made to detect natural gas, which comprises methane, oxygen and nitrogen, are "one-shot" affairs. That is, one takes the unit to various localities, say within a plant, turns the machine on and takes a reading. All of these devices fall into the category of "detectors."
U.S. Pat. No. 2,533,339, Willenborg, describes a combustible vapor detector consisting of a metal block having a reference and a sensing chamber with a separate metallic filament located in each. The sensing chamber is an open communication with a passage through the block which allows air which normally flows through the block and may or may not contain combustible vapors to come in contact with the filament in the sensing chamber. A wire mesh screen across the bottom of the sensing chamber acts as a flame arrestor. A wheatstone bridge, which is normally in a balanced condition, is connected to both filaments. If combustible vapor comes in contact with the sensing filament the resistance thereof is changed, the bridge becomes unbalanced, and an alarm is set off. Also a fan is energized that evacuates the air surrounding the apparatus and thereby clears the area of combustible vapors. The Willenborg device has many shortcomings. The detection time is relatively poor since air samples are not forced past the sensing filament but are just allowed to normally flow through the apparatus without any bias being applied thereto. This can result in a mixture of air and combustible gas which may not be detected until an explosive mixture is formed. Another negative feature of the Willenborg apparatus is that turbulence in the air samples which reach the sensing filament can give rise to false alarms being set off. In addition, variations in the resistance of the filament for reasons other than those associated with combustible vapor can easily trigger false alarms since the wheatstone bridge is relatively easily unbalanced.
U.S. Pat. No. 3,087,795, Ross, teaches the use of an instrument which is unbalanced for a zero combustible gas concentration, but becomes balanced for a specific gas concentration. This kind of unbalanced instrument could be used with the Willenborg apparatus to help reduce false alarms.
The use of a porous diffuser element behind and completely covering the mesh wire screen of the willenborg sensing chamber would substantially eliminate most of the turbulence in samples of air which would reach the sensing filament. This would also further limit false alarms. The addition of pumping air samples past the above-modified sensing chamber would, to some extent, improve the response time. The response time would still be relatively poor because of the relatively slow process of diffusion. This problem could be attenuated if air samples were pumped solely through the wire mesh and diffuser and then forced to exit at some point in the sensing chamber after the sensing filament. This approach would limit air turbulence and produce a faster flow of samples of air past the filament. One problem with this solution is that the diffuser element would have to be relatively large to enable a sufficiently large amount of air to be pumped through the sensing chamber. The large size of the diffusion element would be difficult to achieve economically.
The flow of air over a platinum filament causes the filament to slowly age, i.e., oxidation occurs which increases the resistance of the filament. The greater the flow of air over a platinum filament the shorter the useful lifetime thereof. The resulting increase in resistance of the filament makes frequent readjustment of the apparatus necessary. Thus it is undesirable to have a large flow of air over the filament because of the shortened lifetime of the filament and the need for frequent readjustment of the apparatus.
It would be desirable to have a combustible gas detector that has a relatively fast detection time, provides continuous monitoring, is very sensitive to the presence of combustible gas or vapor, is relatively insensitive to turbulence in the air being sampled, is relatively insensitive to electric or temperature changes caused by effects other than combustible vapor or gas, is not in need of frequent readjustment, and is economically within the reach of most residential homeowners.